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Auto Layout Guide

from https://developer.apple.com/library/ios/documentation/userexperience/conceptual/AutolayoutPG/AutoLayoutConcepts/AutoLayoutConcepts.html

Auto Layout Concepts

  • Solve a system of constraints to determine your layout.
  • Every constraint is of the form y = m*x + b, where y and x are attributes of views, and m and b are floating point values.
  • Use attributes leading and trailing instead of left and right to deal with RTL languages.
  • Constraints are cumulative, and do not override each other. You must remove the first constraint manually.
  • Intrinsic content size is when a leaf-level view, such as a button, knows more about its size than the code that positions it. The element will grow and shrink appropriately with different content.

Working with Auto Layout Programmatically

  • In the visual format string, - denotes a standard Aqua space, which is 8.0 between sibling views and 20.0 between a view and its superview.
  • The NSDictionaryOfVariableBindings method creates a dictionary where the keys are the same as the corresponding value’s variable name.
  • If the bounds transform of a container changes, then a space like -80- scales too.

Resolving Auto Layout Issues

  • Auto Layout issues occur when there are too few constraints, or the system contains conflicting or ambiguous constraints.

Content Size Is Undefined

  • Container views may use their content to determine their own size at runtime.
  • Placeholder constraints address this, such as a minimum width for a view. This placeholder is removed at build time, when your view defines its size.

The Size of Custom View Content Is Unknown

  • Unlike standard views, custom views have no defined intrinsic content size.
  • Use a placeholder intrinsic content size to indicate the custom view’s content size. Again, this is removed at runtime.

Debugging in Code

  • Method _subtreeDescription of NSView creates a textual description of the view hierarchy.
  • Call method constraintsAffectingLayoutForAxis: to get constraints affecting a particular orientation. The debugger can print their details.
  • On OS X, you can visualize the constraints by calling visualizeConstraints:.

Auto Layout by Example

Using Scroll Views with Auto Layout

  • The size of the content inside of a scroll view is determined by the constraints of its descendants.
  • You must make sure you create constraints for all the subviews inside a scroll view.
  • There cannot be any missing constraints, starting from one edge of the scroll view to the other.

Spacing and Wrapping

  • The height of the spacer views can be any value, including 0. However, you must create constraints for the height of the views.

Implementing a Custom View to Work with Auto Layout

  • A custom view class must provide enough information so that the Auto Layout system can make the correct calculations and satisfy the constraints.

A View Specifies Its Intrinsic Content Size

  • The intrinsicContentSize method of a view tells the layout system that it contains content that the layout system doesn't understand, and provides to the layout system the intrinsic size of that content.
  • Method intrinsicContentSize can return absolute values for its width and height, or return NSViewNoInstrinsicMetric for either value to indicate no intrinsic metric for a dimension.
  • For example, a button returns an intrinsic size with a width and height large enough not to clip the text or image. A horizontal slider has an intrinsic height, but no intrinsic width.

Views Must Notify Auto Layout If Their Intrinsic Size Changes

  • If the content of a view changes, which in turn changes its intrinsic size, then it must call invalidateIntrinsicContentSize.

Appendix A: Visual Format Language

  • You cannot express a fixed aspect ratio using the visual format language; you must instead create a NSLayoutConstraint using constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:.